While a number of film structures can be sealed to a container to form a package, the majority of these structures do not open by the delamination mechanism of this invention.
In the prior art, many of the film structures available peeled by a cohesive split of one of the sealant layers of the lidstock laminates. Prior art sealant layers included a monolayer sealant, such as of a 2.0 mil thickness of a polypropylene/polyethylene (PP/PE) blend. Prior art lidstock laminates also include a multi-layer sealant wherein one of the sealant layers cohesively splits during the opening of the lidstock.
One of the main problems with these prior art film laminates is that they are difficult to remove from the container. It is desirable that the opening of the package not occur between the outer layer of the lidstock sealant layer and the tray. This opening mechanism would lend itself too readily to package failures during handling and processing, especially during the demanding processing of retort applications.
It is preferred that the opening mechanism of a film structure/container package be failure within the film structure layers. This insures the hermetic seal of the film structure/container assembly. However, the cohesive split opening mechanism of the prior art requires a relatively large force to open or peel such lidstocks because it requires the physical splitting of one of the layers.
Consumers find the relatively large force requirements for opening such packages to be undesirable. This problem is more severe for consumers who prefer to remove the lidstock from the container while the package is cold. A cold peel of the lidstock requires an even greater amount of physical strength than a hot peel. Most of the available lidstocks must be peeled while the container is hot, such as after the tray/lidstock package is placed in the microwave oven, because of the great difficulty of removing these lidstocks when they are cold.
It is desirable to provide a multi-layer structure for which the opening mechanism is an adhesive failure between two predetermined layers. This allows for a more controllable and consistent peeling force.
One known multi-layer structure which delaminates between two specific layers is a lidstock for a polypropylene tray. The sealant layers are polypropylene adjacent the tray, with an ethylene acrylic acid copolymer blended with ethylene methacrylate copolymer (EAA/EMA layer) in the adjacent layer. The remaining layers of the multi-layer laminate are adhered to the EAA/EMA blend layer. When the lidstock is removed from the tray, the lidstock breaks at the interface between the polypropylene layer and the EAA/EMA layer. However, this lidstock is not approved by the FDA for retort applications.
It is believed that the delamination structure of this invention and packages made therefrom, especially multi-layer laminates sealed to containers, and capable of withstanding retort applications have not been disclosed or suggested in the prior art. Retortable packages, such as a multi-layer laminate or lidstock sealed to a thermoformed tray, contain food and must be retorted if they are to be shelf stable, i.e., having a shelf life of at least six months without refrigeration. The retort process subjects the sealed package to a temperature of 212.degree. F. to 275.degree. F. for 20 to 60 minutes or perhaps even longer, up to 100 minutes, depending on the size of the container. During the retort process, gases are generated within the package and pressure increases greatly. Although the retort system may include an over pressure to help balance the package internal pressures, the net result will still be a pressurized package during retorting.
Thus, the multi-layer laminates and also the seals to the container must be sufficiently strong to withstand the increased internal pressure and the elevated temperatures. These retortable packages often require a relatively large force to open the packages after microwave heating, in part because the forces generated in the interior of the package during retorting require a strong seal. The required force is even greater if it is desired to open the package, particularly a retortable package, while it is cold.
There is a great need for a multi-layer laminate which can be sealed to a container to make a package which does not open during the retort process and can be peeled opened both before and after cooking, by average manual force. It is an object of this invention to meet this need.